Hiberfil.sys

hiberfil.sys is the file used by default by Microsoft Windows to save the machine's state as part of the hibernation process. The operating system also keeps an open file handle to this file, so no user, including the Administrator, can read the file while the system is running.

Although most of the data structures required to parse the file format are available in the Microsoft Windows debug symbols, the compression used (Xpress) was undocumented until it was reverse engineered by Matthieu Suiche and Nicolas Ruff. Currently, Sandman is the only open-source tool that can read the Windows hibernation file.